The assignee of the present invention manufactures and deploys communications spacecraft. Such spacecraft operate within a regulatory regime that licenses at least one operating frequency bandwidth for a particular spacecraft communications service and specifies, inter alia, the maximum signal power spectral density (PSD) of communications signals radiated to the ground. A growing market exists for provision of high data rate communication services to individual consumers and small businesses which may be underserved by or unable to afford conventional terrestrial services. To advantageously provide high data rate communication services to such users, the spacecraft must (1) provide a high PSD so as to enable the use of low cost user terminals, and (2) efficiently use the licensed bandwidth so as to maximize the communications throughput for a particular licensed bandwidth.
A typical satellite communications network 100 is illustrated in simplified form in FIG. 1. The system includes a satellite 11, typically though not necessarily located at a geostationary orbital location defined by a longitude. Satellite 11 is communicatively coupled to at least one gateway 12 and to a plurality of user terminals 16. The user terminals 16 comprise satellite terminals that may be handheld mobile telephones or car phones, or may be embedded, for example, in laptop or desktop personal computers, set top boxes or phone booths.
Each gateway 12 and the satellite 11 communicate over a feeder link 13, which has both a forward uplink 14 and a return downlink 15. Each user terminal 16 and the satellite 11 communicate over a user link 17 that has both a forward downlink 18 and a return uplink 19. A spacecraft antenna subsystem may provide an antenna beam pattern wherein an entire service region is covered using the available bandwidth a single time. Advantageously, however, multiple satellite antenna beams (or cells) are provided, each of which can serve a substantially distinct cell within an overall service region.
Dividing the overall service region into a plurality of smaller cells permits frequency reuse, thereby substantially increasing the bandwidth utilization efficiency. Although frequency reuse in this manner is known (see, for example, Ames, et al., U.S. patent application Ser. No. 10/940,356), systems like the one described in Ames require that a total bandwidth allocated to the downlink be divided into separate non-overlapping blocks for the forward downlink 18 and the return downlink 15. Similarly, prior art solutions divide the total bandwidth allocated to the uplink into separate non-overlapping blocks for the forward uplink 14 and the return uplink 19.